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How do I determine the right fuse size for a wire? Answered

I'm working on restoring a robot and one of the ground wires is completely fried. I will replace the wire, but I want to add a fuse to make sure it doesn't happen again. How should I determine the fuse size for the wire?


This robot is a heavy duty one that runs off two small motorcycle batteries. It has two (I think stepper) motors for propulsion, and more servos for pivoting and such. It was made in the mid eighties, so it uses mainly relays for computation. What is really confusing is the fact it has at least 20 different fuses built in, but all are not burnt. So I'm trying to figure out why the wire fried in the first place. Thanks for your input!

That it has a bank of fuses should come as no surprise. "systems", as opposed to single circuits, often use a variety of protective elements to ensure (or attmept to ensure) that a failure in that subsystem will not compromise the integrity of the entire assembly. In the industrial systems I've designed or otherwise been involved with, circuit breakers, fuses, and overvoltage protections add up quickly....nearly every sub circuit has some sort of protection either inbuilt or on its feeder. Pretty standard mode of operation and often required by regulations.

Do you know the gauge of the wire that got burned up? The gauge may give us a clue...although a first glance suggests that the drive motors or the battery feeds themselves may be the cause of the burned out wire..ie a simple short....for instance, if the positive lead shorted to the robot chassis, presuming that the "ground" is normally connected to both the negative terminal of at one of the batteries and also to the chassis; a good guess for many systems including robots.

It's also quite possible that one (or even both) of the propulsive motors failed, leading to an over-current situation at stall, and which may not have been properly protected or someone may have carelessly used to large a fuse prior to the failure.

Secondly, which fuses are/were blown? and which sub-systems do they feed? That's another good place to start looking for damage, whether primary (the cause) or secondary (a result of the failure).

choosing the right fuse for a circuit is dependent on several factors
Three that come to mind immediately are
1 source voltage (AC? DC? voltage?)
2. operating current
3. load type (motor? instrumentation? etc.)

 Would it be possible for you to provide more details about the circuitry being protected? Selecting the right type and value of a fuse isn't just a matter of current. For instance, choosing a fast blo fuse would be the wrong decision for a motor, whereas choosing a slo-blo might ruin instrumentation. In fact, you may require several fuses to protect the components properly.

Note that for motors, a rule of thumb for selecting the current rating for a fuse is equal to the fault current...175% FLA (full load amps), where the next highest available fuse value is chosen. Wiring for the motors is done using a 135% of expected load, then choose gauge to pull at least that value. This is from National Electric Code, although my direct read on NEC is 2 years out of date at this point.

As other comments, you go on the current. What should it need max (amps)? Choose a fuse not much more but no less than that.


All good advice. But just so you know, you never need to buy a low voltage fuse. Might as well buy 120V fuses, as they are often cheaper and can be used for any voltage up to 120V. At a lower voltage, they work just the same.

The only time you would necessarily need a lower voltage fuse is for smaller physical size. :)

1 more thing: in a robot, I presume you have motors. Anytime you have inductive loads, like you get when you switch a motor, you get current surges that can kill a fast-blow fuse. So you're looking for a slow-blow fuse.

get a small fuse, if it blows under normal operating conditions, get a larger fuse

I have not wired a robot, but will try to help. I hope someone with specific knowledge answers you, but this is a start. 

It is important to try and figure out why the ground wire is fried.  It sounds like there is a short somewhere or a burned out component.  Whatever the problem is should be fixed.

If you are satisfied this is fixed, measure the current being draw with an ammeter when everything is running, motors, lights, etc.  If the batteries are supplying, say, 6 v, and the system is drawing 2 amps, then you would use a slightly larger fuse, say 3 amps, 6 v.  If you don't have an ammeter, just start with a low value fuse, say 1 amp, and increase it to 2 amps if the 1 amp blows, etc.    

Check out this robot fuses tutorial at http://www.societyofrobots.com/electronics_fuse_tutorial.shtml 

Hope this is a help.